Focus On...the Sharp-shinned and Cooper's Hawks
Every fall San Diego birders welcome the return of the hawks of the genus Accipiter with mixed emotions. It is always a thrill to see them as they dash through the trees after prey or as they cruise down the ridge of Point Loma, heading south. With them though, they bring one of the trickiest identification problems among North American birds.
The Sharp-shinned and Cooper's Hawks offer a marvelous example of parallel evolution. The adults of each are extremely similar, both with red eyes, gray upperparts, finely rusty-banded underparts, and a broadly gray-banded tail. The immatures likewise are closely similar, with yellow eyes, brown upperparts, heavily brown-streaked underparts on a whitish background, and the tail banded in shades of brown. Because of the two species' similarity in plumage, differences of size and shape are the most useful features in identification. Unfortunately, the difficulty of assessing distance to the bird and evaluating its posture and angle with respect to the eye, especially in a quick view, means that even experienced observers are unable to identify some individuals in the field. Always try to get a prolonged view, study the birds to gain experience, and don't rush to judgment.
The next step in identifying accipiters, after distinguishing adult from immature, is to understand the birds' sexual dimorphism. Females of both species are much larger than the males, by about 13% to 19% in linear measurements and 65% to 70% in weight. Thus the common problem is whether a bird is a large female Sharp-shin or a small male Cooper's. The male Sharp-shin is roughly the size of an American Robin, the female Sharp-shin and male Cooper's the size of a Rock Dove, and the female Cooper's the size of an American Crow. There is minimal overlap in wing length, though, female Sharp-shins going up to 217 mm and male Cooper's down to 214. In the field body bulk is more obvious than a linear measurement, and there is no overlap in weight. Female Sharp-shins weigh up to around 225 grams, while male Cooper's average about 280 and go down to around 245 grams. For comparison, the average weight of a White-tailed Kite (a species with little or no sexual dimorphism) is around 322 grams. So an accipiter notably smaller than the kite is a Sharp-shin, one near the same size or larger is a Cooper's.Because both species, especially the shy Sharp-shin, are seen in flight more often than perched, familiarity with differences in their flight silhouettes is essential to their identification. The wings of Cooper's are shorter and more rounded in proportion to the bird's body; the wings of the Sharp-shin are slightly more pointed and proportionately longer, even though the bird is smaller. The head of Cooper's Hawk is slightly larger in proportion to the bird's body, the Sharp-shin's smaller. The longer wings of the Sharp-shin are held slightly forward, making the bird look like it has no neck; the Sharp-shin's small head appears to protrude less from the body than Cooper's does. When the bird is perched, the Sharp-shin's head looks small and rounded, the Cooper's blocky and often triangular.
The differences between the two species in tail shape are useful, but it's important to understand how the shape changes as the tail is spread or folded. If the Sharp-shin's tail is folded, it looks more or less square, with the outermost feather only being slightly shorter than the others. In Cooper's the two outermost pairs of rectrices are clearly shorter than the others. When the tail is spread, as it frequently is in flight, the Sharp-shin's can look quite rounded, though less so than Cooper's. Only with experience watching the birds can you learn how the apparent shape changes with the birds' posture and actions. Beware that an accipter's tail looks more square the more it is viewed from the side. Cooper's tail has a broader, whiter pale tip, the Sharp-shin's a narrower, grayer one. The difference is most pronounced in fall soon after the feathers have been replaced in the single annual molt. As the months go by, the tip is worn down and the difference between the two species is reduced.
A couple of differences in plumage color are worth mentioning. The brown streaks on the immature Sharp-shin may be tinged reddish; those of Cooper's are dark brown to blackish. If the reddish tinge can be seen, it suggests the Sharp-shin, but it is often not obvious. The barring on the underparts of adults of both species, of course, is rufous. The crown of the adult Cooper's is dark, contrasting with the buffy nape. The crown of the adult Sharp-shin is gray, not contrasting with the gray nape. Of course, seeing the crown when the bird is flying or perched high in a tree is difficult, and feathers viewed edge on look darker than feathers of the same color viewed perpendicularly.
Cooper's Hawk is quite noisy in spring, calling "kek-kek-kek...," in a series of variable length. This call instantly distinguishes Cooper's from the Sharp-shin. The Sharp-shin's call is much higher pitched and rarely if ever given away from its breeding range.
The Sharp-shinned Hawk is a winter visitor to San Diego County, while the Cooper's Hawk is with us year round. In fall, the Sharp-shin returns usually in the middle of September and is most numerous as a fall migrant in late September and early October, when up to 15 in a day can be seen migrating down Point Loma. Through the winter it remains widespread but in low density. The distribution we have observed over the past four years shows the Sharp-shin scarcer in the Anza-Borrego Desert than on the coastal side of the mountains, but it can be missed even in well-covered coastal areas. Clearly the birds prefer trees to hunt among, but otherwise they have no specialized habitat preference. Four in a day is the maximum count in our winter database, by John Dietrick in East San Diego (R10) on 20 December 1997. Evidently they use urban areas just as readily as natural habitats.
In spring the Sharp-shinned Hawk begins migrating out in March, and after the first week of April it is rare. The latest records in our atlas database are from 26 April, when noted north of Pala (C11) by John & Beverly Hargrove, near Scissors Crossing (J22) by Ed Hall, and in Hauser Canyon (T21) by Jeff Wells. In 1982 Richard Webster reported one from Point Loma on 28 April. We have a specimen in the collection dated "May 1979," but the date's lack of specificity may betray some uncertainty.
The old literature contains a couple of reports implying nesting of the Sharp-shinned Hawk in San Diego County's mountains, but there is no proof. Jon Dunn saw an apparent pair in Banner Canyon near Julian on 29 April 1978, and Roger Higson saw a single bird on Palomar Mountain on 14 July 1982. Though the Sharp-shin has nested south to the San Jacinto Mountains it is rare there, and it may not nest regularly anywhere in southern California. Certainly the main breeding range is far to the north of us. There is no clear evidence of changes in the numbers of Sharp-shinned Hawks in western North America, and there is no suggestion that the species is enjoying an increase like the Cooper's Hawk's.
In the late 1970s Jon Dunn commented to me that he thought most Cooper's Hawks reported on Christmas bird counts were misidentified Sharp-shins. Today, the larger species outnumbers the smaller even as a winter visitor. The maximum count of Cooper's Hawks reported per square in a day in winter goes up to seven [in north Borrego Springs (F24) on 19 December 1999 during an Anza-Borrego Christmas bird count], that for the Sharp-shin only to four. The map of the species' winter distribution shows that Cooper's Hawk is widespread in San Diego County but more numerous in the coastal lowland than in the mountains. In the Anza-Borrego it occurs primarily at oases and developed areas, rarely being picked up in natural open desert.
As a species originally attached to oak woodland the Cooper's Hawk's primitive breeding distribution probably coincided closely with that of the Acorn Woodpecker or Oak Titmouse, withdrawing from the coast toward the south. With the extensive planting of trees in what was once treeless scrub, the hawk now breeds out to Point Loma and the Tijuana River valley. On the east side of the mountains Cooper's Hawks nest in the few canyons that support significant riparian woodland, such as Alder Canyon (C11), where Paul Jorgensen found a used nest on 20 June 2001, and San Felipe Creek near Scissors Crossing (J22), where Mark Jorgensen found a nest with nestlings on 28 June 2000.
One unexpected result of our atlas effort, though, was the number of Cooper's Hawks scattered around the desert in habitat seemingly unsuitable for their breeding. Some of these must have been late spring migrants, but this seems especially unlikely for birds at Lower Willows in Coyote Creek Canyon (D23) on 30 May 2001 (Mel Gabel) and in south Borrego Springs (G24) on 21 May 2000 (Bob Th?riault) and 20 July 1999 (Paul Jorgensen). On 29 April 1997 Mark Jorgensen saw two Cooper's Hawks carrying prey near Vallecito Stage Station (M25), hinting at nesting. Finally, John Fitch confirmed the species' first known nesting on the floor of the desert on 31 May 2001 when he found an adult feeding nestlings in an orange grove along DiGiorgio Road in the northern Borrego Valley (E24).
Cooper's Hawk has always been a regular nesting species in San Diego County. In the past, it was linked closely with oak woodland, and the densely foliaged crowns of the coast live oak remain a favored site for Cooper's Hawks to place their nests. Concern over possible population declines led the California Department of Fish and Game to include Cooper's Hawk on its list of "bird species of special concern" in 1976. In my 1984 "Birds of San Diego County" I called it rare in summer. It became one of the species "covered" under San Diego's multiple-species conservation plan.
In the 1980s, though, Cooper's Hawks began adapting to the urban environment, nesting in eucalyptus trees in Balboa Park and spreading throughout the city. In the 1990s this adaptation accelerated and the birds' numbers increased conspicuously. By the time we initiated the atlas in 1997, Cooper's Hawks had colonized many small parks and schoolyards in inner-city San Diego: Roosevelt Junior High School, the Educational Cultural Center, Emerald Hills Park, Kimball Park in National City, Eucalyptus Park in Chula Vista. At the same time reports of nests in suburban and rural areas proliferated. Eucalyptus trees have become as common a nest site as coast live oaks.
How does a bird so suddenly depart from the conditions to which it has adapted for millennia? In some cases the species was preadapted to the conditions that urbanization presents. We can see this easily with the House Finch, Mockingbird, and Anna's Hummingbird, for example, which move into new urban areas as soon as they are built. In these species, the new habitat must agree with what the birds are programmed by instinct to consider acceptable. In some cases like the Cooper's Hawk, though, I suspect that the suitability of a new habitat must be learned. For decades Cooper's Hawks were regarded as "chicken hawks" and persecuted as predators of poultry. Even San Diego's pioneer naturalist Frank Stephens, while supporting the protection of raptors that feed more heavily on rodents, said in 1919 of Cooper's Hawk that it "deserves no mercy." Under these conditions the birds were surely conditioned to avoid people and retreat from the advance of gun-toting settlers.
A shift in society's attitudes, though, coincided with the maturation of urban trees over many square miles of coastal southern California. Though the occasional gunslinger may still take a shot at a Cooper's Hawk, collisions with picture windows are now the most serious source of mortality the species faces at the hand of man. I suspect that once enough birds learned that people rarely pose a threat any longer, the barrier to their occupying a habitat to which they were preadapted fell. The open "woodland" of a eucalyptus-planted park, school campus, or neighborhood is just as suitable for Cooper's Hawks as a secluded oak groveespecially when the numbers of prey are inflated by a steady supply of picnic scraps or bird feeders. Clearly any intent of the multiple-species conservation plan to address Cooper's Hawks is now misguided.
Philip Unitt and Nicole Perretta